Photolithography, also termed as “optical lithography” or “ultraviolet (UV, including deep ultraviolet, i.e. DUV, and extreme ultraviolet, i.e. EUV) photolithography,” is a process used in microfabrication to pattern parts of a thin film onto a substrate. It uses light to transfer a desired geometric pattern from a “photomask” (or simply “mask”) to a light-sensitive chemical “photoresist” (or simply “resist”) onto a wafer substrate in an exposure pattern. A series of chemical treatments can then engrave the exposure pattern into, or enable deposition of a new material in the exposure pattern onto, the material underneath the photoresist. Photolithography can create small features (down to a few tens of nanometers in size), and is a cost-effective and reliable patterning technology in fabrication of very-large-scale integration (VLSI) devices down to sub-10 nm technology nodes. In integrated circuits (ICs) fabrication, a modern complementary metal-oxide-semiconductor (CMOS) wafer will go through photolithographic cycles for many times (e.g., 50 times) before a functional IC is formed.
During a photolithography process, light is shone onto a mask pattern which makes an imprint on a resist coated over a silicon wafer. The proper functioning of the circuit on the silicon wafer depends on the fidelity of transferring this pattern. Ideally, an output circuit patterned on the wafer is the same as the mask pattern. However, the imaging system is band-limited and can introduce distortions caused by diffraction effects of a lens-projection system, particularly when working at higher resolutions. For example, when wavelength of the light is 193 nm, with technology nodes of, such as, 45 nm, 32 nm or smaller, diffraction is bound to happen in a photolithography system. Thus, techniques are needed to remedy problems posed by sub-wavelength photolithography, such as optical proximity correction (OPC) or other resolution enhancement technologies (RET).
In addition, as demands for smaller and more uniform photomask features is rapidly increasing, the complexity of patterns is also increasing with the need for OPC and RET. These complex mask features demand higher accuracy in pattern placement and dimensional control.
Photolithography simulations incorporating OPC and RET techniques can be used for increased pattern complexity of mask patterns. In simulations for sub-wavelength photolithography, mask images considering near field effects due to mask topology or topography (“near field image”) can be used.